Typical methods of measuring vibration require placing a device such as a seismometer or microphone with a transmitter, at a location of interest; measuring modulation from solar glints, i.e., when the angle subtended by incident sunlight and a surface normal is equal to the angle subtended by the normal and the reflected light; or using an active detector that either emits light or microwave radiation, and subsequently analyzing the reflected light or radiation. Each of the foregoing methods has an inherent shortcoming which compromises its utility.
More particularly, attaching a device to a location of interest requires physical access in addition to providing a means of extracting information from the site, e.g., also attaching a radio frequency transmitter with a battery that will require replacement, or recovering in situ recording media. Given the celestial movement of the sun, the modulation of solar glints can be measured only briefly when the sun, object of interest, and observer possess a preferred orientation, typically once a day. Active devices reveal both the act of measuring and the location of the observer. Common to all of the foregoing methodologies are inaccuracies introduced by background vibrations.
There is a need in the art for means to measure vibration at a site for which it is difficult or impossible to obtain physical access. It is also desirable to be able to obtain a vibration measurement at different times during the day, on an overcast day, or even using moonlight. Furthermore, situations may call for concealing that a measurement is being taken. Concomitant with all of the foregoing shortcomings is the need to obtain an accurate vibration measurement in the presence of background vibration coupling with the object for which a vibration measurement is sought. The present invention fulfills the aforementioned needs in the art.